The present invention generally relates to packet transmission systems and packet transmission methods, and, more particularly, to a packet transmission system and a packet transmission method that perform downlink packet transmission from a network to a mobile node. The present invention also relates to a packet transmission device, a home agent, a mobile terminal, and an access router, all of which are employed in the packet transmission system. The present invention is aimed at increasing throughput of transmission packets.
As the IP (Internet Protocols)-based services have dramatically increased in recent years, mobile communication systems having higher affinities for IP traffic have been developed. In an IP-based mobile communication system (hereinafter referred to simply as the “packet transmission system”), packets directed to a mobile node (such as a portable telephone device) are transmitted through an access router (a router equipped with devices such as radio transmitters that can perform radio communication with the mobile node) that is situated at such a location as to be communicable with the mobile node.
For instance, when the mobile node moves to an end of an area within which the access router can perform transmission (this type of area will be hereinafter referred to simply as a “cell”), the transmission by the single access router becomes intermittent, resulting in quality degradation. To avoid quality degradation in the conventional packet transmission system, the cells are slightly overlaid on one another, so that a plurality of neighboring access routers can perform transmission to the mobile node when the mobile node exists in the vicinity of a cell edge. A handover technique is also applied to the above method so as to attenuate the quality degradation. Handover techniques can be divided into hard handover techniques and soft handover techniques. In a hard handover technique, an access router that can perform the best communication with the mobile node at a certain time is selected, and communication is performed with the mobile node through the selected access router. In a soft handover technique, on the other hand, a plurality of access routers that can perform transmission to the mobile node are selected, and the identical packets are transmitted to the mobile node through the selected access routers at the same time.
A conventional mobile node can be equipped with a plurality of radio receivers, and can switch the radio receivers according to a radio access method (such as the TDMA method or the CDMA method) that can be used in the area in which the mobile node is located. With the radio multiple receivers, the mobile node has the advantage of being able to perform communication as long as one of the radio receivers is communicable.
In the above conventional packet transmission system, however, the mobile node can receive only one packet at a time, regardless of which handover technique is employed. Accordingly, only one packet is transmitted to the mobile node, even where the access routers can perform packet transmission with high quality. As a result, the throughput in the conventional packet transmission system is always low.
Also, in the conventional packet transmission system, even when the mobile node employs a plurality of radio access methods and therefore more than one radio access methods are available, only one of the radio access methods is selected for transmission at a time, also resulting in a low throughput.